The present invention relates to the electric power supply of light-emitting loads, in particular light-emitting diode (LED) lamps that require remote monitoring.
Light-emitting diode (LED) lamps are becoming more and more popular in automotive traffic lights, railway signal lights and other applications. Their lower power consumption is an attractive feature, but the main reason for their popularity is their long life (100 000 hours) compared to standard incandescent lamps (5 000 hours). Manifestly, these features allow important reduction in maintenance costs.
In certain applications, such as railway signal lights, these lamps may be used, as those skilled in the art would know, for main line signalling and/or grade crossing signalling. Grade crossing signals are usually situated in populated areas such-as road intersections and redundant signals are used. Remote monitoring of the LED lamps in grade crossing signals is therefore not common. Main line signals, on the other hand, can be installed in remote areas, which are not easily accessible. Remote monitoring for checking the integrity of the lamps signals is therefore common practice in order to be able to downgrade the aspect in case of a detected signal failure.
For lamps equipped with standard incandescent bulb, electrical integrity can be easily verified. If the filament of the incandescent bulb is in normal condition, current flows through the bulb according to Ohm""s law (I=V/R). Otherwise, if the filament is open, no current flows through the bulb and it should be replaced.
For LED lamps, however, LED current is controlled by a power supply. Current characteristics are therefore not identical in a LED lamp and in an incandescent lamp. In a LED lamp, alternative current (ac) line voltage is rectified and then converted to a suitable level by a dcxe2x80x94dc (direct current) converter, which also regulates LED current. In case of LED failure, or failure of any other electrical component in the LED lamp, it is possible for the power supply to continue drawing current at or near the nominal current value, even if the LED""s are not emitting any light. Remote monitoring systems could therefore see the LED lamp as functioning correctly when in reality it is not. This situation is not acceptable since it can lead to very hazardous train operations and cause major accidents.
Another problem, related to LED lamps and their power supplies and controllers, is caused by electric components which retain residual voltage differentials after power is removed from the LED lamp. The resulting characteristic is that a LED lamp will effectively light up when the power applied to it reaches a first high level while it will be turned off only when the power reaches a second lower level. The resulting problem is that if a certain power is induced by, for example, other nearby cables, the LED lamp could remain on while in fact it should be off. This could also lead to dangerous situations.
These particularities of LED lamps limit their widespread use in situations where they need to be remotely monitored such as in railway main line signalling applications.
An object of the present invention is therefore to allow LED lamps to become compatible with remote detection systems designed for monitoring of incandescent lamps.
Another object of the invention is to provide LED lamp circuitry which will emulate an incandescent lamp""s behavior upon remote monitoring of the LED lamp.
Yet another object of the invention is to provide a control circuit for enabling/disabling the power supply to LED lamps in relation to the level of the line voltage.
According to the present invention, there is provided a module for powering and monitoring a light-emitting diodes load by means of a power line, the module comprising:
an input power switch circuit having an input connected to the power line for receiving power from the power line and monitoring a voltage magnitude thereof to enable and disable the module according to the voltage magnitude of the power line;
a fuse blowout circuit having a fuse through which the input power switch circuit monitors the voltage magnitude of the power line, said fuse blowout circuit being adapted for blowing out said fuse to disable the input power switch circuit if no current flows through the light-emitting diodes load after a pre-determined time when the input power switch circuit is activated;
a cold filament test circuit having an input connected to the power line for emulating an impedance of an incandescent light during a power stage set-up time during which no current is supplied to the light-emitting diodes load;
a current detector circuit for detecting a current supplied to the light-emitting diodes load and for disabling the fuse blowout circuit and the cold filament test circuit when the current of the light-emitting diodes load reaches a predetermined current level; and
a boost power stage circuit having an input connected to the power line and an output connected to the light-emitting diodes load for powering the light-emitting diodes load.
Preferably, the module further comprises a serpentine trace connected in series with the fuse of the fuse blow out circuit for disabling the input power switch circuit upon physical damage to the serpentine trace.
Preferably, the module comprises an input filter circuit connected between the power line and the input power switch for protecting the module.
Preferably, a dummy load resistor is connected across the power line after the power switch circuit to cancel out a negative slope effect on an input impedance of the module.
Preferably, the module further comprises a start-up circuit having a first input connected to the input filter circuit and a second input connected to the current detection circuit, and having an output connected to the boost power stage circuit for starting up the module.
Preferably, the boost power stage circuit has an output capacitor and the module further comprises a quick-bleeder circuit having an input connected to the output capacitor for forcing the output capacitor to discharge at a faster rate through a shunt resistor when the module is turned off.
The embodiments described herein present the advantage that they permit the use of LED lamps in applications, such as railway signal light applications, where there is a need for remote monitoring of the lamps, while keeping the advantageous features of lower power consumption and longer life.
Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings.